Past Research Projects

These three projects (Lysosomes, Post-Synaptic Densities and GABAergic Networks) were funded by the previous P50 grant cycle. The projects worked together to achieve the goal of translating laboratory discoveries into novel effective clinical therapies. Their unifying theme is a focus on the cell biology of specific neurons, and its disruption in Alzheimer's disease triggered by abnormal forms of Amyloid-β peptide.

Extracellular amyloid plaques are a defining feature of Alzheimer’s disease brain pathology. A poorly understood aspect of these plaques is the massive abundance of lysosomes that selectively accumulate within dystrophic (swollen) axons that pass close to them. This axonal enrichment for lysosomes stands in sharp contrast to their near exclusion from the axons of healthy neurons. Beyond the potentially negative consequences of lysosome dysregulation on neuronal proteostasis and health, the enrichment within endosomes and lysosomes of proteases (β and γ-secretases) responsible for amyloid β (Aβ) peptide production combined with the acidic environment of the lysosomal lumen that could further promote Aβ nucleation and suggests a vicious cycle whereby extracellular amyloid plaques trigger axonal membrane trafficking defects that result in abnormal lysosome accumulation and enhanced production/processing of Aβ.

However, lysosomes also contain other proteases (cathepsins) that contribute to Aβ clearance. Due to these positive and negative contributions of lysosomes to Aβ metabolism, it is not clear whether the highly abundant lysosomes within dystrophic axons exert protective versus destructive effects on neuronal health. Thus, while there is evidence to suggest that modulating neuronal lysosome activity could represent a novel therapeutic opportunity, greater understanding of the functions of lysosomes in this context and of the basic cell biology that underlies this phenotype is required for the identification of the specific and relevant molecular targets. The proposed research builds on the unique experience in neuronal membrane traffic, lysosome homeostasis and in vivo imaging of the PI and co-PIs to address these problems in mouse models that recapitulate this Alzheimer’s disease phenotype. To this end, we propose to: 1) investigate the mechanisms that give rise to the extreme abundance of lysosomes within Alzheimer’s disease dystrophic axons; 2) take advantage of novel imaging-based assays for assessment of amyloid plaque formation and growth to dissect the relationship between amyloid plaque growth and neuronal lysosome function; and 3) evaluate specific strategies that modulate neuronal lysosome function in mice for their ability to protect neurons from the toxicity of abnormal β-amyloid metabolism. These studies will test the hypothesis that abnormal neuronal lysosome functions are an important contributing factor to Alzheimer’s disease pathology and may point to new therapeutic targets for the disease.

Synaptic communication is at the crux of brain function, and a key feature of AD is synaptic malfunction and synapse loss. A range of genetic and biomarker data indicate that the Aß peptide triggers synaptic disease. The mechanisms by which AD-selective forms of Aß damage the synapse are not well defined but have the potential to provide multiple sites for therapeutic intervention, which are distinct from regulating APP and Aß themselves. Numerous studies implicate Fyn kinase in the synaptic pathophysiology of AD, with links to both Aß and Tau pathology. For transgenic AD mice, genetic removal of Fyn kinase alleviates, and overexpression of Fyn exacerbates, the impairment of synaptic density and spatial memory. Our work, confirmed and extended by others, showed that Cellular Prion Protein (PrPC) acts as a cell surface binding site for toxic Aß oligomers. Engagement of PrPC by Aβ was found to activate Fyn kinase, initiating a detrimental signaling cascade with synaptic dysfunction.

Using Fyn and PrPC as “molecular handles” for synaptic pathology at the inner and outer surfaces of the post-synaptic density, we have sought to understand how these proteins are coupled and which signal transduction pathways are dysregulated in AD. Our Preliminary data strongly support a crucial role for mGluR5 and Fyn in AD pathophysiology. Here, we seek to validate the role of an Aßo–PrPC–mGluR5–Fyn pathway in AD, identify biomarkers of this pathology, and explore the utility of mGluR5 agents in AD models. This project will advance the hypothesis that selectively blocking Aßo–mGluR5 signaling provides effective diseasemodifying therapeutic strategy for AD.

Attentional deficits diminish the quality of life for Alzheimer’s disease (AD) patients, however the neural basis for this early impairment is not understood. In mouse models of AD, excitatory neurons are hyperactive near amyloid deposits, which could contribute to defects in information processing underlying behavioral dysfunctions. We hypothesize that hypofunction of cortical GABAergic circuits near amyloid deposits is associated with the aberrant network activity and attentional deficits. To test this hypothesis, we will measure the electrophysiological properties of the major subtypes of GABAergic neurons in an AD mouse model. We will also characterize the dendritic morphologies of the inhibitory interneurons in an AD mouse model and in post-mortem cortical tissue from AD patients. To test the prediction that cholinergic signaling in GABAergic interneurons is compromised, we will measure the ability of acetylcholine to modulate cortical network activity. We will also investigate whether knocking down nicotinic acetylcholine receptors in specific subtypes of GABAergic interneurons can recapitulate the neuropathology.

Finally, we will investigate the behavioral relevance by determining whether activating specific GABAergic neuronal populations in the neocortex can alleviate attentional deficits in an AD mouse model. These experiments leverage a combination of optical, electrophysiological, molecular, and behavioral approaches to tease apart the circuit level neuropathology in an AD mouse model. The results will position GABAergic neurotransmission as a point of integration for the synaptic Abeta and cholinergic theories of AD. A positive outcome will also provide evidence for targeting specific cell types for treating attentional deficits in AD.